A moving strip of metal is submerged in a pool of molten zinc or zinc aluminum alloy in galvanizing and aluminizing processes. As the strip is lowered into the molten metal, it passes beneath a submerged horizontal sink roll to reverse its direction of motion. The sink roll rotates on end trunnions carried by a pair of vertical arms that have their lower portions submerged in the molten metal. Typically, this type of arrangement is illustrated in FIGS. 1 and 2.
Certain conditions reduce the life of the support arms and the trunnions. One condition is that the submerged lower ends of the arms heat up to about 900.degree. F. and become relatively soft. Distortion takes place because the arms are partially submerged. The arm strength is reduced. For this reason, the arms warp either toward or away from the roll ends, because of the upward load on the roll by the moving strip tension, as shown in the dashed lines in FIG. 1. The arms may move either outwardly or inwardly. FIG. 1 is an exaggerated illustration of the warping process.
The horizontal trunnions and the roll bearings initially start out with a line contact as the trunnions are rotated. However, as the heated arms bend from the vertical, the trunnions bend downwardly worsening the loss of their line contact. The resulting point contact causes early bearing wear.
Although the bearing clearance should be about 0.005 to 0.010 inches for the best performance, the tendency is for the user to increase the clearance between the bearing and the trunnion, for example, to a 0.25 to 0.50 inch clearance. This larger clearance prevents seizure of the trunnions and the bearings under this deformation. However, a large clearance increases the distortion and permits dross to build up in the bearings, which in turn causes additional erosion and life reduction. In effect, the bearing is self-destructing. Dross is an alloy of the molten metal.
One solution for extending trunnion life and therefore roll life is to provide a self-aligning bearing which will accommodate the deformation of the trunnions. Such a bearing is described in my U.S. Pat. No. 5,549,393. Self-aligning bearings follow the deformation of the arms and trunnions to maintain a line bearing contact. The trunnion cannot machine the contact surface of the bearing. However, it is not always feasible to use self-aligning bearings.
A solution to this bearing problem for those installations that do not use my self-aligning bearings is an object of this invention. Since part of the problem results from the trunnion rotating while the arm is deforming, I have found that a non-rotating shaft which can axially move and will accommodate arm warping. Clearances are not critical because the shaft is not rotating. The arm, in the preferred embodiment, has a bore that allows for relative axial, angular and radial motion between the bore in the arm and the non-rotating shaft.
A stop plate mounted on the arm has a tongue that is received in an opening in the shaft to prevent rotation of the shaft without imposing bending stresses.
A spacer is shrink-fitted in the roll. The roll and the spacer rotate on the shaft that is maintained straight by the metal strip, and its tension. The metal strip engages the roll as the strip is reversing direction in the molten metal bath. The stresses due to elastic and thermal deformation affecting the running area concentricity and angularity, are so reduced that the clearance can be reduced dramatically, for example, to about 0.010 to 0.020 inches versus 0.250 inches to 0.475 inches used in the current state of the art designs. This small clearance allows the formation of hydrodynamic films to further reduce the coefficient of friction and wear, also limiting the size of the contamination, such as dross, that can enter the bearing contact areas.
The availability of AT-103 series alloys makes it possible to design a self-contained, self-aligning galvanizing roll using roller bearings in molten metal. Because sliding friction is eliminated, this design can carry a very heavy roll, accommodate a very high tension from the strip, and permit very high speeds with negligible friction losses or slip with respect to the strip velocity.
Still further objects and advantages of the invention will become readily apparent to those skilled in the art to which the invention pertains upon reference to the following detailed description.